1. Field of the Invention
The present invention relates to an art for manufacturing semiconductor devices. Specifically, the present invention relates to a plasma processing apparatus and a method for controlling the plasma processing apparatus suitable for carrying out a plasma processing of a semiconductor wafer using plasma.
2. Description of the Related Art
Along with the recent trend of large-scale integration of semiconductor devices, the circuit patterns have become finer and finer, and the demands for accurate dimension processing have become very strict. Further, the wafer diameter has become as large as 300 mm with the aim to reduce manufacturing costs of the semiconductor devices, so there are also demands for uniformizing the plasma in a large area from the center of the wafer to the outer circumference thereof in order to enable uniform and high quality processing and to thereby improve the yield factor. Simultaneously, there exists demands for reducing contaminants and to reduce the contamination of wafers caused by metal materials etc. that are generated and scattered by sputtering.
A prior art method that achieves these objects is disclosed in Japanese Patent Application Laid-Open Publication No. 2001-127045 (patent document 1), which discloses a parallel plate plasma generating apparatus comprising an upper metallic plate electrode and a lower wafer (that functions as an electrode), in which RF biases of the same frequency are applied to the upper electrode and the lower electrode (wafer), realizing a method for generating a uniform plasma by controlling the phase of the RF biases.
Further, Japanese Patent Application Laid-Open Publication Nos. 2002-184766 (patent document 2) and 2004-111432 (patent document 3) disclose methods for improving the uniformity of plasma, reducing the amount of contaminants and reducing the charging damage by controlling the phase of the RF between upper and lower biases so that either the upper electrode or the lower electrode constantly functions as an earth.
On the other hand, as disclosed in Japanese Patent Application Laid-Open Publication No. 2001-338917 (patent document 4), it is known that with respect to the influence that an RF transmission path has on the voltage, current and phase difference of RF, the RF waveforms differ between the wafer and the output portion of an RF matching network, so it is effective to use a wafer potential probe to directly measure the wafer potential in order to acquire information on the wafer potential.
However, according to the prior art, there are a means for detecting and a means for adjusting the phase difference of the RF between upper and lower biases, and though it is stated that the phase difference should preferably be controlled to 180 degrees, there is no disclosure on how to guarantee the achievement of the object by phase control. For example, it is stated in the prior art that the object is to improve the uniformity of plasma, but the actual means for guaranteeing the uniformity of plasma via phase control is not made clear. The same can be said for reduction of the generation of contaminants and charging damage.
The second drawback of the prior art is that even if the prior art method can be adopted to control the phase difference to 180 degrees, for example, it is not capable of guaranteeing that the phase difference of the voltages actually appearing at the upper and lower electrodes is 180 degrees. According to the methods mentioned above, the phase data is taken at the matching network of the circuit for applying RF biases to the upper and lower electrodes. However, the RF applied to the upper and lower electrodes undergo various conditions after exiting the output unit of the matching network. Regarding the bias RF application circuit leading to the upper electrode, the RF exiting the output unit of the matching network passes an RF filter, and a separate RF circuit for generating plasma exists as a load of the bias RF circuit, and an RF transmission path leading to the upper electrode is unique to the upper electrode. On the other hand, regarding the bias RF application circuit leading to the lower electrode, a structure for mounting the wafer on the lower electrode and controlling the wafer temperature exists within a transmission path of the RF bias, an electrostatic chucking circuit exists as a load of the RF bias transmission path, and a mechanism for carrying the wafer and a cover protecting the same etc. exists as earth of the RF bias transmission path. Therefore, even if phase data is taken at each matching network of the upper and lower electrodes, the RF transmission paths that lead to the electrodes from the matching network vary, so there is no guarantee that the phase difference of the voltages generated at the electrodes is as controlled.
Another fact that influences the present problem is that the impedance of the RF transmission path leading to the earth seen from the upper electrode and the impedance of the RF transmission path leading to the earth seen from the lower electrode are not equal. The upper electrode sees as a part of the load the vacuum vessel wall, the lower electrode and the electric circuit connected thereto as earth, but the lower electrode sees the vacuum vessel wall, the upper electrode and the electric circuit connected thereto as a part of the load. As described earlier, the structure of the upper and lower electrodes and the electric circuits connected thereto are not equal, so the RF impedances of these loads obviously differ. Further, the area of the vacuum vessel wall seen by the upper electrode and that seen by the lower electrode are not equal, since the upper and lower electrodes are positioned in confronting relationship. Moreover, during processing, the plasma is generated near the upper electrode and is spread out toward the lower electrode, during which the property thereof is varied. Further, since wafer processing is progressed near the lower electrode, the large amount of reaction products generated from the wafer causes the status of plasma near the lower electrode to change drastically.
Accordingly, the impedance determined by the density and electron temperature of plasma near the upper electrode is not equal to the impedance determined by the density and electron temperature of plasma near the lower electrode. Since the upper and lower electrodes see via the plasma having different impedance an earth that is not of the same range, the impedances naturally differ.
Even further, as disclosed in patent document 2, when a magnetic field is applied to a part of or a whole of the vacuum vessel, the influence of this magnetic field causes the impedances of various RF paths leading from the upper and lower electrodes to the earths to differ, since a completely uniform magnetic field does not exist. It may be necessary to vary the various conditions of plasma when processing the wafer (such as the density, the electron temperature, the RF power generating the plasma, the gas species and gas pressure, the positional relationship of the upper and lower electrodes, and magnetic field conditions), which also causes the impedances of the various RF paths from the upper and lower electrodes to the earth to differ.
The phase shift caused by these power paths depend on the stray capacitance and coil component existing on the power paths, so the phase shift increases as the frequency becomes higher. Though it depends on the arrangement of the apparatus, in general, the effect of phase shift becomes significant at a frequency of 1 MHz or higher.
What is concluded from the above is that the phase difference of voltages generated at upper and lower electrodes is determined by the overall circuits of RF passing the electrodes and the frequency of the RF, and therefore, it is not possible according to the prior-art monitoring method to accurately control the phase difference during wafer processing.
The third drawback of the conventional method results from the fact that the voltage waveform of RF biases generated at upper and lower electrodes is not a sine wave, and the conventional methods were incapable of performing optimum value control. It is well known that an electrically nonlinear region called an ion sheath is generated on the front side of the electrode exposed to plasma. Nonlinear means that the current and voltage are not in proportional relationship, wherein the electron current flows into the electrode when the voltage is positive, and this voltage-current property of the electron current depends on the diffusion coefficient of the electron, which normally is the exponential function of the voltage. On the contrary, if the voltage is negative, the ion current flows into the electrode, but this current does not depend on voltage since it is a space-charge limited current, and is substantially constant. When RF is applied to this electrode, current will not flow in proportion to the RF voltage, and the voltage waveform will be deformed from the sine waveform, containing harmonics.
What is important with respect to the present invention is that since the properties of plasma differ at the surface of the upper and lower electrodes, the properties of the ion sheaths in these areas differ, and as a result, the amounts of distortion of the RF at the upper and lower electrodes (the degree and size of the harmonics) vary, or in other words, the waveforms of the voltages of electrodes vary. Actually, as disclosed in patent documents 2 and 3, the measured values of the physical quantities being the index of phase control, which are the maximum electrode voltage, the voltage charged on gate oxide film, δVdc, and the peripheral ion saturation current density, are not optimized at 180 degrees which is the characteristic phase difference, but is displaced therefrom. The cause of this displacement is that the phase difference of the electrodes are not performed accurately since the impedances of the RF transmission paths differ, and since the current/voltage waveform of the electrodes are deformed due to harmonics.
The fourth drawback of the conventional art is that the apparatus for improving wafer processing via phase control is restricted to a parallel plate plasma source equipped with upper and lower electrodes. Various type of plasma sources are used in the industry such as an inductively coupled plasma source or an ECR plasma source, but the methods disclosed in the prior art cannot be applied to these plasma sources since they do not have two independent electrodes that are capacitively coupled with plasma.